1. Field of the Invention
The present invention relates to a hypodermic syringe and more particularly, to a safety hypodermic syringe, which automatically receives the needle assembly inside the plunger by means of a vacuum to prevent contamination after the service of the safety hypodermic syringe.
2. Description of the Related Art
FIGS. 1A and 1B show the operation of a safety hypodermic syringe according to the prior art. According to this design, the safety hypodermic syringe comprises a barrel 1, which has a spring hook 2 extended from the outside wall and projecting into the inside, a hollow cylinder 3, which is inserted into the barrel 1 from the rear side and has a stop member 4 protruded from the periphery near the rear side, a needle assembly 7 fastened to the front side of the hollow cylinder 3 and extended out of the front side of the barrel 1, a tip protector 8 capped on the needle assembly 7, a plunger 6 inserted into the hollow cylinder 3 from the rear side, and a compression spring 5 mounted inside the barrel 1 and stopped between the front side of the barrel 1 and a part at the periphery of the hollow cylinder 3. After the service of the safety hypodermic syringe, the spring hook 2 is turned outwards and disengaged from the stop member 4, for enabling the compression spring 5 to push the hollow cylinder 3 backwards, and therefore the needle assembly 7 and the tip protector 8 are received with the hollow cylinder 3 inside the barrel 1. This structure of safety hypodermic syringe has numerous drawbacks as follows:
1. The compression spring 5 tends to be covered with rust if the safety hypodermic syringe is not used within a short period after its fabrication.
2. The spring hook 2 may be disengaged from the stop member 4 accidentally before the service of the safety hypodermic syringe; thereby causing the parts of the safety hypodermic syringe fall apart.
3. The safety hypodermic syringe is reusable after its service, and an evil person may recycle the used safety hypodermic syringe for a repeat use.
4. The complicated structure of the barrel 1 and hollow cylinder 3 greatly increases the manufacturing cost of the safety hypodermic syringe.